Contaminated soil that is contaminated by a heavy metal or the like may exist at, for example, abolished plant sites that have been in operation since the times when the health hazards due to the heavy metal or the like were not recognized. Moreover, the heavy metal or the like may also be contained in surplus soil for landfill, which is attributable to the contaminated soil. In recent years, it has become known that the heavy metal or the like contained in such soil elutes and intrudes into ground water to cause a situation where human health is threatened, and the importance of the technology to stably immobilize the heavy metal or the like in soil has been recognized. It is desired that the surplus soil used for landfill be neutral (specifically, pH of 5.0 to 9.0) from environmental consideration, however since strongly basic soil as a result of being mixed with mortar is frequently discharged, the surplus soil for landfill is basic in many cases. Furthermore, the surplus soil used for landfill has a high water content in many cases, such as a large amount of sludge that is produced by waste water treatment and water-containing mud including surplus soil waste from construction, and when such soil is used for landfill, it becomes necessary to solidify mud in many cases.
As mentioned previously, from the necessity for preventing the elution of the heavy metal or the like from the soil at abolished plant sites etc. or from the soil filled up with surplus soil and the intrusion of the heavy metal or the like into ground water, various types of insolubilizing materials for a heavy metal or the like have conventionally been used for the purpose of suppressing the elution of the heavy metal or the like, and the effects thereof have been confirmed. It is to be noted that the “heavy metal or the like” in the present invention means the “Heavy Metal or the like being Designated Hazardous Substance (class II designated hazardous substance)” specified in Article 2 of Soil Contamination Countermeasures Act that came into effect in 2003, and specifically denotes the following compounds.                Cadmium and compounds thereof        Hexavalent chromium compounds        Cyanides        Mercury and compounds thereof (including alkyl mercury compounds)        Selenium and compounds thereof        Lead and compounds thereof        Arsenic and compounds thereof        Fluorine and compounds thereof        Boron and compounds thereof.        
As mentioned previously, various proposals have been made for the purpose of insolubilizing the heavy metal or the like contained in soil, and also have been put into practical use. For example, as a technology for making fluorine hardly soluble to stabilize against fluorine-contaminated soil or the like, a method for making fluorine hardly soluble to stabilize by producing fluorinated apatite and ettringite with a material containing gypsum, lime, ferrous sulfate, and a phosphate compound is proposed (see Patent Literature 1). Moreover, there is also a proposal (see Patent Literature 2) on a technology in which the heavy metal or the like is insolubilized by producing ettringite making use of a solidifying material composition comprising cement, aluminum sulfate, and gypsum. Furthermore, the present applicant also has so far made a proposal (see Patent Literature 3) on a gypsum-based insolubilizing and solidifying material for a heavy metal or the like: that makes it possible to make the treated matter after treatment neutral; and that makes it possible to solidify a heavy metal or the like contained in mud concurrently with insolubilizing the heavy metal or the like to impart the strength to the mud, thereby making the mud excellent in handling, by using the gypsum-based insolubilizing and solidifying material for a heavy metal or the like added to and mixed with the sludge produced by waste water treatment or the mud such as surplus soil waste from construction. Specifically, the proposal has been made on an insolubilizing and solidifying material for a heavy metal or the like in which an aluminum compound selected from aluminum hydroxide and so on and a neutralizing agent containing a calcium or magnesium component are contained in calcined gypsum.
Many of the heavy metal-insolubilizing materials that have conventionally been known use a strongly basic material, or a mixture of a strongly basic material and an acidic material, and therefore the treated matter during the treatment process exhibits a strong basicity, however, in a table test, the treated matter that is to be finally obtained becomes neutral because contact with air can be made and favorable mixing among materials can be conducted (see Patent Literature 4).